Lubricants are mainly used to eliminate contact between two elements submitted to sliding movement or lamination. Their main applications are in automotive engines, industrial gear boxes, turbines, compressors and hydraulic systems. Global consumption of lubricants is approximately 37.5 millions of tons annually, being the automotive market the main consumer, with a participation of 54%, followed by the industrial market with a 32%.
Conventional lubricants are composed by a great diversity of chemical additives for enhancing and/or incorporate new properties. The content of additives may vary from a concentration of 10% to 20% by lubricant oil volume. Some of the most common additives are those used for enhancing the viscosity, anticorrosive, antioxidant, detergent, dispersant, anti-wear, crystallization inhibition indexes, for allowing operation at high temperatures and extreme pressures, etc. however, said additives have the inconvenient that they are constituted by sulphates, phosphates and chlorates, which are highly damaging compounds for the environment, besides being very expensive and being technically limited. That is, in some cases, such kind of additives are required to be operated within a limited temperature and/or specific conditions to be activated and to adequately function.
On the other hand, highly efficient industries such as the automotive industry, aim medium-term goals (5-10 years) at reducing by 18% the losses due to friction between main components such as the engine. If such goals are achieved, a worldwide fuel consumption reduction is expected in the order of 41 thousand millions of liters of fuel, as well as a reduction of 101 millions of tons of CO2 to the environment.
Due to the above, it has aroused interest in the scientific community to find low cost and environmentally friendly alternative additives, for which nano additives are an option with great potential for this purpose. There have been reported numerous works about the development of lubricants having nano additives. Below are mentioned the most important.
US patent application No. 2011/8076809 of Tingler et al., describes a submersible electric motor filled with nano fluid as coolant. The base fluid used was a synthetic oil, and the nanoparticles used were selected from the group comprising carbon nanotubes, graphite, diamond, silicon dioxide, aluminum oxide, beryllium oxide, nitride and combinations thereof. The concentration of nanoparticles ranged from 5% to 30% by volume.
US patent application No. US-2004/6828282B2, describes am additive for gasoline and lubricants based on carbon nanotubes. The concentration used was of between 0.01% to 15% by weight, with an aspect ratio of between 5:1 to 100:1, preferably of between 5:1 to 100:1. The reported benefits comprise an increment of the combustion capacity of the gasoline, as well as an increment of the viscosity for the lubricant fluid.
Patent application No. US-2008/7449432B2 of Lockwood, describe an industrial oil added with nanoparticles derived from carbon having a size of between 50 nm to 500 nm at a concentration of between 0.01% to 15% by weight. Besides, conventional additives were added to the nano lubricant, such as detergents, dispersants, antioxidants, etc. The reported benefits comprise viscosity, thermic conductivity and friction coefficient improvements.
International patent application No. WO-2011/046524 A1 of Akin et al. describe a nano additive for lubricant oil and gasoline based on hexagonal boron nitride nanoparticles, boron diamond and boron nitride having a size of 1 nm to 5 nm. The resultant nanolubricant. The resulting nanolubricant further contained at least one conventional anti-friction or anti-wear type additive. Reported benefits comprise improved gasoline combustion capacity as well as reduction in the friction and wear coefficient for use in the lubricating fluid.
US patent application No. 2011/7994105 B2, describe a nano additive for automotive lubricants based on ceramic nanoparticles, silicates, cubic boron nitride and diamond, having a size of between 20 nm to 40 nm, and concentrations of 0.02% to 10% by weight. The reported benefits comprise a reduction in friction coefficients and improvements in fuel efficiency of around 35% for a variety of vehicles.
US patent application No. US-2007/0158609 A of Hong et al. describe a process for preparing a stable suspension of nano carbon particles. The nanoparticles were selected from a group comprising diamond, graphite, fullerenes, carbon nanotubes and a combination thereof, at a concentration of between 0.1% to 30% by weight. The refrigerant oil used as base fluid showed improvements in its rheological, thermal and tribological properties.
US patent application No. US-2010/0029518 A1 of Markovitz et al. describe a nano additive for an automotive and industrial lubricant based on carbon and diamond nanoparticles. The used concentration of nanoparticles was of 10 ppm to 500 ppm by weight, additionally, the composition contains from 200 ppm to 2000 ppm of water.
Finally, US patent application No. US-2011/0046027 A1 of Aruna Zhamu et al, describe a lubricant composition based on mineral oil or synthetic oil incorporating graphene nanoflakes in a dispersed form and at a proportion of 0.001% to 60% by weight. These graphene nanoflakes are preferably single layered and depending on their amount, the lubricant tends to be a fat.
The mineral oil or synthetic oil modified with these graphene nanoflakes has better thermal conductivity, better friction coefficient, a better anti-wear performance and better viscosity stability compared with mineral oils or synthetic oils that are only modified with graphene nanoparticles or carbon nanotubes.